This invention relates to a telephone line interface for data access arrangements (DAA). Specifically, it relates to a line powered DAA with enhanced stability.
The telephone lines to a residence in the United States and elsewhere can have common mode voltages of over 100V, and the FCC requires the telephone lines to be isolated from any electric main powered device (such as a PC) connected to the telephone lines (through a modem for example) to prevent damage to the telephone network. 47 CFR 68.302,4 (Oct. 1, 1997 Edition). A data access arrangement (DAA) is specified by the FCC to isolate the telephone lines from electric main powered devices, such as illustrated in FIG. 4. Since a voice band modem signal is limited to the 50 to 4000 Hz band, a DAA can be constructed using a transformer which operates as a bandpass filter to isolate the electric main powered device from the telephone lines.
A smaller size and potentially lower cost solution uses active circuits to communicate with the central telephone office and various modulation techniques to couple the DAA through small capacitors to a device, such as a PC.
FIG. 5 shows a known line powered telephone line interface circuit for modulating a data signal onto a telephone line using active circuits. The circuit is disclosed and described fully in U.S. patent application Ser. No. 09/028,061 filed on Feb. 26, 1998, entitled xe2x80x9cLow Noise Line Powered DAA With Feedback,xe2x80x9d assigned to the same assignee as the present application, and incorporated herein by reference.
The main function of the circuit is to take the incoming current ILINE supplied by the telephone company and modulate it with a data signal developed by processing a differential data signal source VD with a line modulator so as to place the data signal on the telephone line. The circuit uses transistor Q1 as a line modulator, and contains a shunt regulator in series with the line modulator Q1. A sense resistor RS is placed in series between the line modulator Q1 and the shunt regulator to monitor the current through the shunt regulator.
The circuit depicted in FIG. 5 works by monitoring the current IS through sense resistor RS with a feedback loop around the amplifier A. Resistors RT and RB sense the differential voltage across RS. By setting RT=RB, the current through RT and RB will accurately model the current through RS. The desired signal to be modulated is introduced by a differential signal source VD. The differential signal is created by adding signal VD/2 to common mode voltage, VCM, to create VP, and subtracting VD/2 from VCM to create VN. This differential signal then drives the input resistors RIP and RIN to provide a differential input current signal. The generation of the differential current signal is well known in the art. The control amplifier operates to force the current through resistor RS to equal the desired signal current by regulating transistor Q2 to control the base of transistor Q1, which in turn regulates the current through the collector-emitter path of transistor Q1 and thereby through resistor RS. In this circuit, the collector current of transistor Q1 is controlled by the control amplifier A.
Ideally, the current through RS would equal the current, ILINE, introduced to the system by the telephone company. However, this is not the case in actuality. The current from the telephone company is introduced to the system through the emitter of transistor Q1 (hereinafter xe2x80x9cIE1xe2x80x9d). In the circuit depicted in FIG. 5, IE1 is equal to ILINE, the resistances of RT and RB are a couple hundred thousand ohms, and the resistance of RS is 10-20 ohms. Because of the relatively high level of resistance of RT1 and RB1, the current that flows through RT1 and RB1 can be neglected in the circuit analysis. As current flows through the circuit, IE1 is divided into the transistor Q1 base current (hereinafter xe2x80x9cIB1xe2x80x9d) and the transistor Q1 collector current (hereinafter xe2x80x9cIC1xe2x80x9d) The collector current IC1 through the resistor RS is used by amplifier A in a feedback loop to modulate the desired signal onto ILINE.
FIG. 6 shows another known line powered telephone line interface circuit for modulating a data signal onto a telephone line using active circuits. The circuit is disclosed and described fully in U.S. patent application Ser. No. 09/280,473 filed on Mar. 30, 1999, entitled xe2x80x9cMethod and Apparatus for Decreasing Distortion in a Line Powered Modulator Circuit,xe2x80x9d assigned to the same assignee as the present application, and is incorporated herein by reference.
As in the circuit described above in reference to FIG. 5, the main function of the circuit in FIG. 6 is to take the incoming current ILINE supplied by the telephone company and modulate it with a data signal developed by processing a differential data source signal VD with a line modulator so as to place the data signal on the telephone line. The circuit uses transistor Q1 as a line modulator, and contains a shunt regulator in series with the line modulator Q1. A first sense resistor RS1 is placed in series between the line modulator Q1 and the shunt regulator to monitor the current through the shunt regulator. In addition, a second sense resistor RS2 is added within the modulator to pick up xe2x80x9cstray componentsxe2x80x9d of line current ILINE which are outside of the feedback path containing the first sense resistor RS1, and incorporate the xe2x80x9cstray componentsxe2x80x9d into an additional feedback path around the amplifier A.
The circuit depicted in FIG. 6 works by monitoring the current through sense resistor RS1 and RS2 with feedback loops around the amplifier A. The method of sensing the current through RS1 and RS2, and for generating the differential signal current is similar to the circuit setup described in reference to FIG. 5. The control amplifier operates to force the sum of the current through resistors RS1 and RS2, and thereby ILINE, to equal the desired signal current by regulating transistor Q2 to control the base of transistor Q1, which in turn regulates the current through the source-emitter path of transistor Q1.
FIG. 7 shows another known line powered telephone line interface circuit designed in low voltage CMOS technology for modulating a data signal onto a telephone line using active circuits. The circuit is disclosed and described fully in U.S. patent application Ser. No. 09/407,444 filed on Sep. 29, 1999, entitled xe2x80x9cPre-Charging Line Modem Capacitors to Reduce DC Setup Time,xe2x80x9d assigned to the same assignee as the present application, and is incorporated herein by reference.
As in the circuits described above in reference to FIG. 5 and FIG. 6, the main function of the circuit in FIG. 7 is to take the incoming current, ILINE, supplied by the telephone company and modulate it with a data signal developed by processing a differential data signal source, VD, with a line modulator so as to place the data signal on the telephone line. The circuit uses transistor Q1 as a line modulator, and contains a shunt regulator in series with the line modulator Q1. A sense resistor RS is placed in series between the line modulator Q1 and the shunt regulator to monitor the current through the shunt regulator. In addition, resistors R1 and R2 and capacitors C1 and C2 are included to set the AC gain of the modulator. Because the circuit is used to control AC and DC characteristics, additional components are required to obtain desired AC and DC values.
To have a low enough frequency response for the full voice band (e.g., down to about 50 Hz and up to about 4 kHz), capacitors C1 and C2 need to be large enough so there is not undue frequency response distortion of the signal. However, larger capacitors take a longer time to charge up, i.e., to finish settling down. The settling time may be as large as 400 ms, for example. During the time that the capacitors are charging, the transient charging current is added to the desired DC current level, causing a large error in the DC line current that lasts longer than a typical setup time limit, i.e., approximately 20 ms. Thus, when the line modulator is powered up, the time constant of resistor-capacitor pairs R1, C1 and R2, C2 cause DC current errors that result in a delayed DC setup time.
The circuit in FIG. 7 overcomes startup difficulties by using two precharge amplifiers A2 and A3 and two switches S1 and S2. Amplifiers A2 and A3 are unity gain amplifiers which are used to precharge capacitors C1 and C2, respectively, during startup. Precharge amplifiers A2 and A3 are configured as voltage followers to equalize nodes 1 and A with each other, and nodes 2 and B with each other, respectively. They are enabled and disabled in accordance with an enable signal EN. The goal is to raise node A up to the voltage at node 1 as quickly as possible, and to raise node B up to the voltage at node 2 as quickly as possible, thereby precharging capacitors C1 and C2. Because no DC current passes through resistors R1 and R2, the voltages at nodes A and B should equalize to those of nodes 1 and 2 very quickly, due to the operation of precharge amplifiers A2 and A3. Amplifiers A2 and A3 are operational when they are enabled by a high enable signal EN. A high EN signal is generated from the beginning of the power up phase to some time period which allows the capacitors to be sufficiently precharged. The precharge operation in enhanced by adding switches S1 and S2, which are also controlled by the EN signal so that they are open during the precharge phase. Switches S1 and S2 are placed in series with resistors R1 and R2 to eliminate them from the circuit during the precharge phase.
Although the circuits depicted in FIG. 5, FIG. 6, and FIG. 7 depict line powered DAAs capable of modulating a data signal onto a telephone line, the stability of the systems require improvement in order to create DAAs that are capable of being used for a wide variety of applications. Instability is introduced by the individual components of the line powered DAAs operating as amplifiers and voltage level shifters. Inherent to amplifiers and voltage level shifters is the potential for phase changes, especially at high frequencies. The phase changes associated with the individual components can produce oscillations in the DAA, which leads to system instability.
The present invention proposes a novel method and apparatus for increasing the stability of a line powered telephone line interface or data access arrangement (DAA). The present invention accomplishes the task of increasing stability by strategically placing capacitance in the line powered telephone line interface or DAA. In a preferred embodiment, the invention enhances the normal operation of known DAAs by inserting additional capacitance into the DAA. In addition to enhanced system stability during normal operation, the present invention provides improved system stability on startup.